On 05/05/2011 11:40 PM, NeilBrown wrote:
> On Fri, 6 May 2011 14:24:34 +1000 NeilBrown<neilb@xxxxxxx> wrote:
>
>> On Tue, 03 May 2011 15:20:46 -0500 Michael Reed<mdr@xxxxxxx> wrote:
>>
>>> Resending to linux-raid.
>>>
>>>
>>> Hi Neil,
>>>
>>> My name is Mike Reed. I work at SGI. I've been looking at a performance
>>> issue with md writes and have tracked it down to the md device not having
>>> a high enough initial max_hw_sectors_kb setting.
>>>
>>> There is code in blk_queue_make_request() which lowers the default value
>>> from INT_MAX to BLK_SAFE_MAX_SECTORS, which is 255. This is generally
>>> lower than all the underlying devices with which I use md.
>>>
>>> As md appears to be a stacking driver, i.e., it calls disk_stack_limits()
>>> for each member of a volume, it would seem reasonable for md to use the,
>>> INT_MAX setting for max_hw_sectors_kb instead of BLK_SAFE_MAX_SECTORS.
>>>
>>> I have tried this, and have observed that md correctly limits the md device's
>>> max_sectors_kb to the value of the underlying devices in my mirror volume.
>>>
>>> Is this the correct way to address this issue?
>>>
>>> Applies to linux-2.6.39-rc4.
>>>
>>> Signed-off-by: Michael Reed<mdr@xxxxxxx>
>>>
>>>
>>> --- drivers/md/md.c 2011-04-18 21:26:00.000000000 -0700
>>> +++ drivers/md/md.c.new 2011-04-21 15:53:11.452536201 -0700
>>> @@ -4328,6 +4328,7 @@ static int md_alloc(dev_t dev, char *nam
>>> mddev->queue->queuedata = mddev;
>>>
>>> blk_queue_make_request(mddev->queue, md_make_request);
>>> + blk_queue_max_sectors(mddev->queue, INT_MAX);
>>>
>>> disk = alloc_disk(1<< shift);
>>> if (!disk) {
>>>
>>
>> Hi Mike,
>> sorry for not responding to this earlier - it seemed to keep falling through
>> cracks for some reason :-(
>>
>> Yes, I completely agree with your analysis and think you patch is correct and
>> useful.
>> I will queue it up for the next merge window.
>>
>> thanks,
>> NeilBrown
>
> ...except of course that it should be
> blk_queue_max_hw_sectors(......)
> ^^
>
> because blk_queue_max_sectors was renamed back in 2.6.34 :-)
>
> NeilBrown
Thank you for catching that! I've attached a corrected patch for the record.
Signed-off-by: Michael Reed<mdr@xxxxxxx>
--- drivers/md/md.c 2011-04-18 21:26:00.000000000 -0700
+++ drivers/md/md.c.new 2011-05-09 10:57:16.000000000 -0700
@@ -4328,6 +4328,7 @@ static int md_alloc(dev_t dev, char *nam
mddev->queue->queuedata = mddev;
blk_queue_make_request(mddev->queue, md_make_request);
+ blk_queue_max_hw_sectors(mddev->queue, INT_MAX);
disk = alloc_disk(1 << shift);
if (!disk) {
--
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